Mental retardation, Down's syndrome, fragile X syndrome and autism are developmental and genetic disorders that affect day to day functioning, including learning, memory, speech, social skills and behavior. Currently available treatment regimens for humans with mental retardation, Down's syndrome, fragile X syndrome and to assist in day-to-day functioning, include, for example, behavioral modifications and treatment with a range of medications including anti-depressant and anti-psychotic drugs. However, such regimens frequently are not effective or may produce undesirable side-effects with long term use, particularly the use of anti-psychotic drugs. Thus, there is a need to develop new, improved and effective methods to treat mental retardation, Down's syndrome, fragile X syndrome and autism.